Teens and Toddlers is an intervention that enables young people at risk of teenage pregnancy to spend time mentoring a toddler as well as participating in counselling and classroom work focused on youth development. This trial aims to test primary hypotheses that girls age 13/14 who are randomly allocated to participate in the T&T intervention are more likely to report use of contraception at last sex, less likely to report frequent sex without contraception over the previous 3 months, less likely to expect to become teenage parents and more likely to report high scores for youth development indicators one year after the intervention than are similar girls who are randomised not to participate. The trial involves a total of 180 girls per arm and is being conducted across England funded by the Department for Education and conducted by the London School of Hygiene and the National Centre for Social Research.